Behold: A Smartly Built Basket of Growing Mid-Caps

Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some mid-cap stocks to your portfolio because they're big enough to have proven themselves to some degree and yet small enough to still have lots of room to grow, the Guggenheim S&P MidCap 400 Equal Weight ETF (NYS: EWMD) could save you a lot of trouble. Instead of trying to figure out which companies will perform best, you can use this ETF to invest in lots of them simultaneously.

It's noteworthy, too, that the index weights its holdings equally, instead of letting the higher market-cap companies dominate it, as many other indexes do. (The famous S&P 500 index, for example, is market-cap-weighted.)

The basicsETFs often sport lower expense ratios than their mutual fund cousins. The mid-cap ETF's expense ratio -- its annual fee -- is a relatively low 0.41%. The fund is very small, too, so if you're thinking of buying, beware of occasionally large spreads between its bid and ask prices. Consider using a limit order if you want to buy in.

With a low turnover rate of 7%, this fund isn't frantically and frequently rejiggering its holdings, as many funds do.

What's in it?Plenty of mid-cap companies had strong performances over the past year. Omega Healthcare Investors (NYS: OHI) , for example, surged 22%. The real estate investment trust lends money, rents properties, and is poised to benefit from an influx of Americans with new health insurance, thanks to President Barack Obama's reforms. The nation's aging population also bodes well for Omega and its long-term-care facilities.

Other companies didn't do as well last year, but could see their fortunes change in the coming years. Oil and gas specialist Quicksilver Resources (NYS: KWK) shrank by 69%, having taken on a lot of debt to fund drilling for natural gas, which is currently experiencing very low prices. The stock got a boost from rising oil prices, but it has also been downgraded by analysts worried about negative free cash flow, among other things.

Tempur-Pedic International (NYS: TPX) , down 55%, took a big hit when management tempered expectations for the coming year. It has actually been posting an accelerating revenue growth rate, though, and robust earnings growth. Some now see it as a bargain at recent levels. A recovering housing market will help prop up shares of this company.

Intrepid Potash (NYS: IPI) , down 32%, is in the fertilizer business, and has suffered from weak pricing in its market. It's the largest U.S. fertilizer producer, and stands to benefit from growing domestic demand. Corn, in particular, is being planted in vast quantities, and it's particularly fertilizer-dependent.

The big pictureA well-chosen ETF can grant you instant diversification across any industry or group of companies -- and make investing in and profiting from it that much easier.